Optical scanning device and image forming apparatus

ABSTRACT

An optical scanning device includes a coupling optical system, a light source including a plurality of light emitting units for emitting light beams, and a deflector including a deflecting surface for deflecting the light beams. The coupling optical system is arranged on an optical path between the light source and the deflector so that the light beams enter the deflector at an angle with respect to a normal direction of the deflecting surface in a sub-scanning direction. The light emitting units are arranged two-dimensionally, and a distance between two light emitting units at opposite ends in a main scanning direction is smaller than a distance between two light emitting units at opposite ends in the sub-scanning direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. Ser. No.12/025,343, filed Feb. 4, 2008, now U.S. Pat. No. 7,924,487, whichclaims priority to Japanese priority document 2007-029874, filed inJapan on Feb. 9, 2007, and 2007-050884, filed in Japan on Mar. 1, 2007,the entire contents of each of which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning device and an imageforming apparatus.

2. Description of the Related Art

Laser image forming apparatuses have been widely used forelectrophotographic image recording. Such an image forming apparatusincludes an optical scanning device, and a method of rotating a drum toform a latent image, while scanning laser beams, using a polygon scanner(for example, a polygon mirror) in an axial direction of the drum havingphotosensitivity has been generally used. In the field ofelectrophotography, image forming apparatuses are required to form animage with high write density at a high speed to improve image qualityand operability.

As one method of realizing both high density and high speed, it can beconsidered to rotate a polygon scanner at a high speed. In this method,however, noise and vibration in the polygon scanner increase, powerconsumption also increases, and durability is deteriorated.

As another method of realizing both high density and high speed, it canbe considered to use multi beams of light emitted from a light source.

As one example of a multi-beam lighting system, a lighting system inwhich a plurality of semiconductor lasers and coupling lenses providedin a pair with each semiconductor laser are arranged in a main scanningdirection, and these are integrally supported has been proposed. Thislighting system can decrease the size of a deflection-reflecting surfaceby crossing light beams in the main scanning direction near thedeflection-reflecting surface of a deflector that deflects the beamsfrom semiconductor lasers, and can reduce a difference in opticalperformance between the beams, because the beams after being deflectedcan be made to pass through substantially the same optical path of animaging optical system. Such a multi-beam lighting system is alsoreferred to as a “crossing multi-beam light source”. Accordingly,because a cheap semiconductor laser is used, and fewer components areused, a very cheap multi-beam lighting system and optical scanningdevice can be provided.

For example, Japanese Patent Application Laid-open No. H11-340570discloses a photoelectric converter, which includes on the samesubstrate a plurality of photoelectric conversion units including aplurality of laminated semiconductor layers and having a side at an endin a direction perpendicular to the lamination direction.

Japanese Patent Application Laid-open No. H11-354888 discloses asemiconductor light emitting diode including an arrangement substrate,one substrate supported by the arrangement substrate, a plurality oflight emitting units each including a plurality of semiconductor layersformed by being laminated on the side of the substrate opposite thearrangement substrate, a plurality of ohmic electrodes provided on theside of the light emitting units opposite the substrate and electricallyconnected in one-to-one correspondence with each light emitting unit,and at least one heat release layer formed on the side of respectivelight emitting units opposite the substrate by being laminated on thesubstrate via the light emitting units.

However, to use the photoelectric converter disclosed in Japanese PatentApplication Laid-open No. H11-340570 and the semiconductor lightemitting diode disclosed Japanese Patent Application Laid-open No.H11-354888 for the multi-beam light source, the power consumption islarge and a heat output is large. Therefore, the upper limit of thenumber of beams is about four beams or eight beams in view of the cost.Further, because each element has one-dimensional arrangement of thelight emitting units, if the number of beams is considerably increased,a deviation amount of the beams from an optical axis of an opticalsystem increases, and beam characteristic can be deteriorated.

Therefore, a vertical cavity surface emitting laser (VCSEL) has beenproposed, in which a plurality of light emitting points can be easilyformed on one element two-dimensionally, and the power consumption issmall.

Japanese Patent No. 3227226 discloses an image forming apparatus, whichuses an optical scanning device including a light source that emitsindependently modulatable three or more laser beams from a lightemitting unit arranged two-dimensionally, a laser beam deflector thatperiodically deflects the laser beams in respective directions, and ascanning optical system that images the laser beams on a surface to bescanned. In the optical scanning device, if it is assumed that thenumber of the laser beams is n, number of imaging spots formed on animage carrier are S1 to Sn from the top in a direction perpendicular toa scanning direction, and a distance from the imaging spot S1 to theimaging spots S2 to Sn respectively measured at right angles to thescanning direction is L2 to Ln, values D2 to Dn respectively obtained bydividing L2 to Ln by a scanning line pitch p are approximately integralnumbers, and surpluses M2 to Mn as a result of dividing D2 to Dn by nare natural numbers less than n, which are different from each other.

Japanese Patent Application Laid-open No. 2004-287292 discloses anoptical scanning device having a first substrate fitted to a housingfixed to an image forming apparatus body, on which a light emittingdiode and a driving circuit therefor are mounted, a second substratefitted to the housing away from the first substrate, on which aconnector for connecting a harness from the image forming apparatus bodyis mounted, and an elastically deformable connection member thatelectrically connects a first substrate terminal with a second substrateterminal.

Japanese Patent Application Laid-open No. 2005-250319 discloses alighting system formed by combining a plurality of sets of a lightsource in which a plurality of independently modulatable light emittingpoints are arranged two-dimensionally and a coupling lens that couplesdivergent beams emitted from the light source, an optical scanningdevice with the lighting system, and an image forming apparatus with theoptical scanning device.

Generally, in the optical scanning device, to suppress occurrence ofdensity unevenness on an image due to a change in a quantity of lightwith temperature fluctuation and change with lapse of time, auto powercontrol (APC) is executed, by which the quantity of light emitted fromthe light source is monitored by a detector such as a photodiode, and anoutput level is controlled based on the result. In this case, with anedge emitting laser, the beams are emitted in two directions forward andbackward. Therefore, if the beam emitted forward is used for scanningand the beam emitted backward is used for monitoring, even if the beamfor scanning is returned to the light source, there is little influenceon the monitoring result. However, with the VCSEL, because the beams areemitted only in one direction, the emitted beams need to be divided ordiverged, to use one for scanning and the other for monitoring. In thiscase, return light to the light source can affect the monitoring result.

Recently, an optical scanning device using an overfilled optical systemhas been proposed, in which a width of a deflector surface of adeflector that deflects a plurality of light beams is smaller than anirradiation range of a plurality of light beams. Such an opticalscanning device is disclosed in, for example, Japanese PatentApplication Laid-open No. 2003-270577. By the overfilled optical system,the width of the deflector surface can be reduced, and therefore moredeflector surfaces can be provided than in the conventional deflectorhaving the same diameter (diameter of a circle centering on a rotationaxis, with which the deflector touches internally). Therefore, thescanning speed can be improved without increasing the number ofrevolution of the deflector.

In the optical scanning device using the overfilled optical system,however, there is a problem in that light quantity distribution of thelight beams reflected by the deflector surfaces is not uniform. Further,in the VCSEL arrays, because a divergence angle of the light beam issmall, the light quantity distribution of the light beams incident tothe deflector surfaces is slightly non-uniform as compared with that ofthe edge emitting laser. Therefore, to combine the vertical-cavitysurface emitting type light source with the overfilled optical system, atechnique for equalizing the light quantity distribution of the lightbeams is required.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an aspect of the present invention, there is provided anoptical scanning device. The optical scanning device includes a lightsource that includes a plurality of light emitting units for emittinglight beams; a deflector that includes a deflecting surface fordeflecting the light beams from the light source; a scanning opticalsystem that focuses the light beams deflected by the deflector on asurface to be scanned; and a monitoring optical system that monitors aquantity of light beams emitted from the light source. The light beamsenter the deflector from a direction inclined with respect to a normaldirection of the deflecting surface at least in a sub-scanningdirection. The light emitting units are arranged two-dimensionally, anda distance between two light emitting units at opposite ends in a mainscanning direction is smaller than a distance between two light emittingunits at opposite ends in the sub-scanning direction.

According to another aspect of the present invention, there is providedan optical scanning device that is configured to scan a plurality ofto-be-scanned surfaces with light beams. The optical scanning deviceincludes a light source that includes a plurality of light emittingunits for emitting light beams; a deflector that includes a deflectingsurface for deflecting the light beams from the light source; aplurality of scanning optical systems that each correspond to one of theto-be-scanned surfaces and focus the light beams deflected by thedeflector on corresponding one of the to-be-scanned surfaces; and amonitoring optical system that monitors a quantity of light beamsemitted from the light source. The light beams enter the deflector froma direction inclined with respect to a normal direction of thedeflecting surface at least in a sub-scanning direction. The lightemitting units are arranged two-dimensionally, and a distance betweentwo light emitting units at opposite ends in a main scanning directionis smaller than a distance between two light emitting units at oppositeends in the sub-scanning direction.

According to still another aspect of the present invention, there isprovided an image forming apparatus that includes an image carrier andan optical scanning device that scans a surface of the image carrierwith light beams based on image data. The optical scanning deviceincludes a light source that includes a plurality of light emittingunits for emitting light beams; a deflector that includes a deflectingsurface for deflecting the light beams from the light source; a scanningoptical system that focuses the light beams deflected by the deflectoron a surface to be scanned; and a monitoring optical system thatmonitors a quantity of light beams emitted from the light source. Thelight beams enter the deflector from a direction inclined with respectto a normal direction of the deflecting surface at least in asub-scanning direction. The light emitting units are arrangedtwo-dimensionally, and a distance between two light emitting units atopposite ends in a main scanning direction is smaller than a distancebetween two light emitting units at opposite ends in the sub-scanningdirection.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an image forming apparatus according toa first embodiment of the present invention;

FIG. 2 is a schematic diagram of an example of an optical scanningdevice shown in FIG. 1;

FIG. 3 is a schematic diagram of a two-dimensional array of VCSELs in alight source shown in FIG. 2;

FIG. 4 is a schematic diagram for explaining a structure of each VCSELin the two-dimensional array shown in FIG. 3;

FIG. 5 is an enlarged view of part of the VCSEL shown in FIG. 4;

FIG. 6 is a schematic diagram for explaining an inclination of lightwith respect to a normal direction of a deflection-reflecting surface;

FIG. 7 is a schematic diagram for explaining light emitting units of thelight source;

FIGS. 8A and 8B are schematic diagrams for explaining sags of light fromthe light emitting units in a polygon mirror;

FIG. 9 is a schematic diagram for explaining a sub-scanning beam pitchdeviation;

FIG. 10 is a schematic diagram of a first modification of a monitoringoptical system;

FIG. 11 is a schematic diagram of a second modification of a monitoringoptical system;

FIG. 12 is a schematic diagram for explaining a modification of theVCSEL;

FIG. 13 is an enlarged view of part of the VCSEL shown in FIG. 12;

FIG. 14 is a table of characteristics of the VCSEL shown in FIG. 12;

FIG. 15 is a schematic diagram of an image forming apparatus accordingto a modification of the first embodiment;

FIG. 16 is a schematic diagram of an optical scanning device shown inFIG. 15;

FIGS. 17A to 17C are schematic diagrams for explaining an optical pathof light incident to a polygon mirror and light reflected by the polygonmirror;

FIG. 18 is a side view of the optical scanning device shown in FIG. 16;

FIG. 19 is a schematic diagram for explaining an optical scanning deviceof one-directional scanning type;

FIG. 20 is a schematic diagram of a modification of the optical scanningdevice shown in FIG. 19;

FIG. 21 is a schematic diagram of an optical scanning device accordingto a second embodiment of the present invention;

FIG. 22 is a schematic diagram of a light source of the optical scanningdevice shown in FIG. 21;

FIG. 23 is a sectional view of a VCSEL formed in the light source shownin FIG. 22;

FIG. 24 is an enlarged view of an example of an active layer of theVCSEL shown in FIG. 23;

FIG. 25 is an enlarged view of another example of an active layer of theVCSEL;

FIGS. 26A and 26B are schematic diagrams for explaining a method forequalizing the quantity of light beams incident to a deflector surface;

FIGS. 27A and 27B are schematic diagrams for explaining a position onthe deflector surface where a light beam is incident;

FIGS. 28A and 28B are schematic diagrams for explaining the method forequalizing the quantity of light beams incident to a deflector surface;

FIG. 29 is a schematic diagram for explaining an effect obtained by aline-image forming lens having a refracting power in a main scanningdirection;

FIG. 30 is a schematic diagram for explaining another effect obtained bythe line-image forming lens having a refracting power in the mainscanning direction;

FIG. 31 is a schematic diagram of a modification of an aperture of anaperture plate shown in FIG. 2;

FIG. 32 is a schematic diagram of a multicolor-image forming apparatus;

FIG. 33 is a perspective view of an optical scanning device shown inFIG. 32; and

FIG. 34 is a schematic diagram for explaining optical scanning performedby the optical scanning device shown in FIG. 33.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are explained in detailbelow with reference to the accompanying drawings.

A first embodiment of the present invention is explained below withreference to FIGS. 1 to 9. FIG. 1 is a schematic diagram of a printer1000 as an image forming apparatus according to the first embodiment.

The printer 1000 includes an optical scanning device 1010, aphotosensitive drum 1030, a charger 1031, a developing roller 1032, atransfer charger 1033, a discharge unit 1034, a cleaning blade 1035, atoner cartridge 1036, a feed roller 1037, a feed tray 1038, a pair ofregistration rollers 1039, a fuser roller 1041, an eject roller 1042,and an eject tray 1043.

The photosensitive drum 1030 has a photoconductor layer on its surfaceto be scanned. The photosensitive drum 1030 rotates clockwise (directionof arrow) in FIG. 1.

The charger 1031, the developing roller 1032, the transfer charger 1033,the discharge unit 1034, and the cleaning blade 1035 are arranged nearthe surface of the photosensitive drum 1030. The charger 1031, thedeveloping roller 1032, the transfer charger 1033, the discharge unit1034, and the cleaning blade 1035 are arranged in this order in arotation direction of the photosensitive drum 1030.

The charger 1031 uniformly charges the surface of the photosensitivedrum 1030.

The optical scanning device 1010 irradiates light modulated based onimage information from a host apparatus (for example, a personalcomputer) onto the surface of the photosensitive drum 1030 charged bythe charger 1031 to form a latent image corresponding to the imageinformation thereon. The latent image is moved toward the developingroller 1032 with the rotation of the photosensitive drum 1030.

The developing roller 1032 allows toner supplied from the tonercartridge 1036 to adhere to the latent image formed on the surface ofthe photosensitive drum 1030 to obtain a toner image. The toner image ismoved toward the transfer charger 1033 with the rotation of thephotosensitive drum 1030.

A recording medium (sheet) 1040 is stored in the feed tray 1038. Thefeed roller 1037 is arranged near the feed tray 1038, and takes out thesheet 1040 one by one from the feed tray 1038 and carries it to theregistration rollers 1039. The registration rollers 1039 is arrangednear the transfer roller 911, to temporarily hold the sheet 1040 takenout from the feed roller 1037, and also sends off the sheet 1040 towarda space between the photosensitive drum 1030 and the transfer charger1033, matched with the rotation of the photosensitive drum 1030.

To electrically attract the toner on the surface of the photosensitivedrum 1030 to the sheet 1040, a voltage of a reverse polarity to that ofthe toner is applied to the transfer charger 1033. The toner image onthe surface of the photosensitive drum 1030 is transferred onto thesheet 1040 by the voltage. The sheet 1040 having the transferred tonerimage is carried to the fuser roller 1041.

Heat and pressure are applied to the sheet 1040 by the fuser roller1041, thereby fixing the toner on the sheet 1040. The sheet 1040 withthe toner being fixed is carried to the eject tray 1043 via the ejectroller 1042 and sequentially stacked on the eject tray.

The discharge unit 1034 discharges the surface of the photosensitivedrum 1030.

The cleaning blade 1035 removes the toner remained on the surface of thephotosensitive drum 1030 (residual toner). The removed residual toner isreused. The surface of the photosensitive drum 1030, from which theresidual toner has been removed, returns to the position of the charger1031 again.

The configuration of the optical scanning device 1010 is explained withreference to FIG. 2. The optical scanning device 1010 includes a lightsource 14 a, a coupling lens 15, an aperture plate 23, a cylindricallens 17, a reflecting mirror 18, a polygon mirror 13 a, a deflector-sidescanning lens 11 a, an image-surface side scanning lens 11 b, an imaginglens 24, and a photodiode 25. The main scanning direction corresponds toY-axis direction, the sub-scanning direction corresponds to Z-axisdirection, and a direction perpendicular to these scanning directionscorresponds to X-axis direction.

The light source 14 a includes, as shown in FIG. 3, a two-dimensionalarray 100 in which, for example, 40 light emitting units 101 are formedon one substrate. The two-dimensional array 100 includes ten lightemitting lines, in which four light emitting units are arranged with anequal interval along a direction forming an angle α of inclination(hereinafter, “T direction”) from a direction corresponding to the mainscanning direction (hereinafter, “M direction”) toward a directioncorresponding to the sub-scanning direction (hereinafter, “Sdirection”). The ten light emitting lines are arranged in the Sdirection with an equal interval. That is, 40 light emitting units aretwo-dimensionally arranged along the T direction and the S direction.

The distance between two light emitting units positioned at the oppositeends in the M direction is 90 micrometers, and the distance between twolight emitting units positioned at the opposite ends in the S directionis 216 micrometers as one example.

Each light emitting unit is a VCSEL of 780 nm (nanometer) band, and, asshown in FIG. 4, semiconductor layers such as a lower reflecting mirror112, a spacer layer 113, an active layer 114, a spacer layer 115, anupper reflecting mirror 117, and a p-contact layer 118 are sequentiallylaminated on an n-GaAs substrate 111. The one in which a plurality ofsemiconductor layers are laminated is hereinafter refereed to as a“laminated body”. FIG. 5 is an enlarged view around the active layer114.

The lower reflecting mirror 112 has 40.5 pairs of ann-Al_(0.9)Ga_(0.1)As low refractive index layer (low refractive indexlayer 112 a) and an n-Al_(0.3)Ga_(0.7)As high refractive index layer(high refractive index layer 112 b). The refractive index layers are setto have an optical thickness of λ/4 in which λ denotes an emissionwavelength. A composition gradient layer (not shown), in which thecomposition is gradually changed from one composition to the othercomposition, is provided between the low refractive index layer 112 aand the high refractive index layer 112 b for reducing electricalresistance.

The spacer layer 113 is an Al_(0.6)Ga_(0.4)As layer.

The active layer 114 includes, as shown in FIG. 5, an Al₀₁₂Ga_(0.88)Asquantum well layer 114 a and an Al_(0.3)Ga_(0.7)As barrier layer 114 b.

The spacer layer 115 is an Al_(0.6)Ga_(0.4)As layer.

The part including the spacer layer 113, the active layer 114, and thespacer layer 115 is referred to as a resonator structure, and thethickness thereof is set to have an optical thickness of one wavelength(the wavelength λ is 780 nanometers) (see FIG. 5).

The upper reflecting mirror 117 has 24 pairs of a p-Al_(0.9)Ga_(0.1)Aslow refractive index layer (low refractive index layer 117 a) and ap-Al_(0.3)Ga_(0.7)As high refractive index layer (high refractive indexlayer 117 b). The refractive index layers are set to have an opticalthickness of λ/4. The composition gradient layer (not shown), in whichthe composition is gradually changed from one composition to the othercomposition, is provided between the low refractive index layer 117 aand the high refractive index layer 117 b for reducing the electricalresistance.

An AlAs selected oxide layer 116 is provided at a position λ/4 away fromthe resonator structure in the upper reflecting mirror 117.

A manufacturing method of the two-dimensional array 100 is brieflyexplained below.

(1) The laminated body is created according to crystal growth usingmetal organic chemical vapor deposition (MOCVD method) or molecular beamepitaxy (MBE method).

(2) A slot is formed around each region, which becomes the lightemitting unit, by dry etching to form a “mesa”. An etching bottom faceis set to reach the inside of the lower reflecting mirror 112. Theetching bottom needs only to exceed at least the selected oxide layer116. Accordingly, the selected oxide layer 116 appears on a side wall ofthe slot. Further, it is preferred that the size (diameter) of the mesais 10 micrometers or more. If the mesa is too small, heat can beaccumulated at the time of operation of the device, to negatively affecta light emitting characteristic. Further, it is preferred that the widthof the slot is 5 micrometers or more. It is because if the width of theslot is too narrow, etching control becomes difficult.

(3) The laminated body with the slot formed thereon is heat-treated inwater vapor, to selectively oxidize part of the selected oxide layer 116in the mesa, thereby changing the part to an Al_(x)O_(y) insulatorlayer. At this time, an unoxidized AlAs region in the selected oxidelayer 116 remains at a central part of the mesa. Accordingly, currentbottleneck structure is formed, which restricts a route of a drivingcurrent for the light emitting unit to only the central part of themesa.

(4) An SiO₂ protective layer 120 having a thickness of, for example, 150nanometers is provided, excluding a region where an upper electrode 103is formed and a light emitting unit 102 in each mesa, and a polyimide119 is buried in each slot for planarization.

(5) The upper electrode 103 is formed on a region excluding the lightemitting unit 102 on the p-contact layer 118 in each mesa, to form eachbonding pad (not shown) around the laminated body. A wiring (not shown)for connecting each upper electrode 103 with the corresponding bondingpad is formed.

(6) A lower electrode (n-side common electrode) 110 is formed on theother side of the laminated body.

(7) The laminated body is cut into a plurality of chips.

Referring back to FIG. 2, the coupling lens 15 substantially collimatesthe light emitted from the light source 14 a.

The aperture plate 23 is arranged on an optical path between thecoupling lens 15 and the cylindrical lens 17, and has an aperture forspecifying a beam diameter of the light via the coupling lens 15 atleast in the sub-scanning direction. Because the aperture plate 23 usesthe light reflected around the aperture for monitoring, the apertureplate 23 is arranged inclined with respect to a virtual surface verticalto a traveling direction of the light from the light source 14 a towardthe polygon mirror 13 a. Accordingly, it can be prevented that the lightreflected around the aperture returns to the light source 14 a. Thelight from the polygon mirror 13 a toward the light source 14 a can bealso shaded.

The cylindrical lens 17 is arranged on the optical path between theaperture plate 23 and the reflecting mirror 18, and images the lighthaving passed through the aperture in the aperture plate 23 in thesub-scanning direction, near the deflection-reflecting surface of thepolygon mirror 13 a via the reflecting mirror 18.

A soundproof glass 21 is arranged between the cylindrical lens 17 andthe polygon mirror 13 a, and between the polygon mirror 13 a and thedeflector-side scanning lens 11 a.

The optical system arranged on the optical path between the light source14 a and the polygon mirror 13 a is also referred to as a couplingoptical system. In the first embodiment, the coupling optical systemincludes the coupling lens 15, the aperture plate 23, the cylindricallens 17 and the reflecting mirror 18.

The polygon mirror 13 a has a quaternary mirror, and each mirror becomesthe deflection-reflecting surface. The polygon mirror 13 a rotates atequal velocity around a rotation axis parallel to the sub-scanningdirection.

The deflector-side scanning lens 11 a is arranged on the optical path ofthe light deflected by the polygon mirror 13 a.

The image-surface side scanning lens 11 b is arranged on the opticalpath of the light via the deflector-side scanning lens 11 a.

The optical system arranged on the optical path between the polygonmirror 13 a and the photosensitive drum 1030 is also referred to as ascanning optical system. In the first embodiment, the scanning opticalsystem includes the deflector-side scanning lens 11 a and theimage-surface side scanning lens 11 b. An optical system, whichcorresponds to the VCSEL, in which a lateral magnification in thesub-scanning direction does not change in the main scanning direction,is used for the scanning optical system.

As one example, as shown in FIG. 6, optical axes of the coupling lens 15and the cylindrical lens 17 are arranged inclined with respect to anormal direction of the deflection-reflecting surface of the polygonmirror 13 a in the sub-scanning direction. That is, the coupling opticalsystem is a grazing incidence optical system. For simplicity, in FIG. 6,the optical path from the reflecting mirror 18 toward the polygon mirror13 a is rotated to match the X-axis direction.

Accordingly, the light from the light source 14 a is incident to thedeflection-reflecting surface as being inclined with respect to thenormal direction of the deflection-reflecting surface in thesub-scanning direction (grazing incidence). In this case, it can beprevented that the light reflected by the deflection-reflecting surfacereturns to the light source side. When the optical axes of the couplinglens 15 and the cylindrical lens 17 are arranged to match the normaldirection of the deflection-reflecting surface, the light emitted fromthe light source 14 a enters the deflection-reflecting surface parallelto the normal direction of the deflection-reflecting surface, and thelight reflected by the deflection-reflecting surface can return to thelight source side.

The angle of the grazing incidence by the coupling optical system (theangle of inclination with respect to the normal direction of thedeflection-reflecting surface) is set in a range of from 0.5 degree to1.0 degree. Because the angle of grazing incidence is small,deterioration of the beam spot diameter by wave front aberration andcurvature of the scanning line seldom occur. Further, it is advantageouswhen deterioration of the optical performance is corrected by a specialsurface for the scanning lens, which has been proposed by the presentinventors (see Japanese Patent Application Laid-Open No. 2006-72288).

When a light source having a plurality of light emitting units(multi-beam light source), the grazing incidence optical system, and thepolygon mirror are used to scan the same surface to be scannedsimultaneously by a plurality of beams, even with the beams directedtoward the same image height, the angle of rotation of the polygonmirror is different according to the position of the light emittingunit. Therefore, a deviation between image heights with an interval oftwo scanning lines adjacent to each other (sub-scanning beam pitch) inthe sub-scanning direction on the surface to be scanned (hereinafter,“sub-scanning beam pitch deviation”) can occur due to an influence of anoptical sag.

This phenomenon is explained by the light from the two light emittingunits (light emitting units v1 and v2 shown in FIG. 7) away from eachother in the M direction in the two-dimensional array 100. The lightfrom the light emitting unit v1 and the light from the light emittingunit v2 enter the deflection-reflecting surface from differentdirections on a plane perpendicular to the sub-scanning direction.Therefore, to deflect the light from the light emitting unit v1 and thelight from the light emitting unit v2 to the same image height, theangle of rotation of the polygon mirror needs to be different. At thistime, because the rotation axis of the polygon mirror (rotation center)is not on the deflection-reflecting surface, the optical sag occurs.

As one example, in FIG. 8A, the sag on the deflection-reflecting surfacewhen the light from the light emitting unit v1 and the light from thelight emitting unit v2 are directed toward the image height of −150millimeters is shown. In FIG. 8B, the sag on the deflection-reflectingsurface when the light from the light emitting unit v1 and the lightfrom the light emitting unit v2 are directed toward the image height of+150 millimeters is shown. In either case, the sag amount is differentbetween the light from the light emitting unit v1 and the light from thelight emitting unit v2. The sub-scanning beam pitches at the position ofthe image height of −150 millimeters and at the position of the imageheight of +150 millimeters are different from each other.

As shown in FIG. 9, light beams directed toward the same image heightare shifted in the main scanning direction due to the influence of thesag, resulting in a difference in the position where the light beamspass through the deflector-side scanning lens 11 a. In the grazingincidence optical system, the scanning beams are curved in thesub-scanning direction and enter the deflector-side scanning lens 11 adue to a difference in the optical path length from thedeflection-reflecting surface to the deflector-side scanning lens 11 a.Therefore, if the beams are shifted in the main scanning direction, therefracting power received in the sub-scanning direction changes, tochange the position of the beam spot in the sub-scanning direction onthe surface to be scanned, and in the multi beams, the sub-scanning beampitch becomes different between image heights, that is, there is adeviation (sub-scanning beam pitch deviation).

However, in the first embodiment, the plurality of light emitting unitsare arranged two-dimensionally, and the distance between two lightemitting units at opposite ends in the direction corresponding to themain scanning direction is smaller than that between two light emittingunits at opposite ends in the direction corresponding to thesub-scanning direction. Accordingly, the sub-scanning beam pitchdeviation can be reduced.

Referring back to FIG. 2, the imaging lens 24 focuses the lightreflected by the aperture plate 23. The photodiode 25 is arranged nearthe light convergence position, and outputs a signal corresponding tothe amount of light received (photoelectric conversion signal). Theoutput signal of the photodiode 25 is used for monitoring the quantityof light emitted from the light source 14 a, and a drive current for thelight emitting units is corrected based on the monitoring result.

In the optical scanning device 1010, the monitoring optical systemincludes the aperture plate 23 and the imaging lens 24.

The quantity of emitted light from each of the light emitting units canbe detected by, for example, a time-division detection method ofsequentially lighting up the light emitting units, or a method in whichat least two light emitting units are set in a group and a plurality ofgroups in various combinations are sequentially lighted up in a group todetect the quantity of emitted light from each group and the quantity ofemitted light from each light emitting unit is calculated based on theresult.

Thus, because the quantity of light emitted from the light emittingunits can be individually known by one monitoring optical system, theoptical system can be simplified and the number of parts can be reduced.As a result, a low-cost and small-sized apparatus can be realized.

As explained above, according to the first embodiment, the opticalscanning device 1010 includes the light source 14 a having a pluralityof light emitting units, the polygon mirror 13 a that deflects the lightfrom the light source 14 a, the coupling optical system arranged on theoptical path between the light source 14 a and the polygon mirror 13 toguide the light from the light source 14 a to the polygon mirror 13 a,the scanning optical system that guides the light deflected by thepolygon mirror 13 a to the photosensitive drum 1030, and the monitoringoptical system that monitors the quantity of light emitted from thelight source 14 a. The coupling optical system is arranged so that thelight from the light source 14 a enters from a direction inclined withrespect to the normal direction of the deflection-reflecting surface inthe sub-scanning direction. Accordingly, light reflected by thedeflection-reflecting surface can be prevented from returning to thelight source side. Thus, the quantity of light emitted from the lightsource 14 a can be accurately monitored.

Because the light source 14 a has the two-dimensional array of theVCSELs, high speed and high density can be realized.

The light emitting units are two-dimensionally arranged so that thedistance between two light emitting units at opposite ends in thedirection corresponding to the main scanning direction is smaller thanthat between two light emitting units at opposite ends in the directioncorresponding to the sub-scanning direction. Thus, the sub-scanning beampitch deviation due to multi beams, which is peculiar to the grazingincidence optical system, can be reduced. As a result, beams can scanthe photosensitive drum 1030 very stably.

Because of the optical scanning device 1010 capable of stably scanningthe photosensitive drum 1030 with light beams, the printer 1000 iscapable of forming a high quality image at high speed.

As shown in FIG. 10, the monitoring optical system can include theaperture plate 23, the reflecting mirror 27, and the imaging lens 24. Inthis case, the aperture plate 23 is arranged so that the light from thelight source 14 a is reflected around the aperture in a −Y direction,and the reflecting mirror 27 is arranged so that the reflected lightfrom the aperture plate 23 is reflected in a −X direction. Accordingly,the light source 14 a and the photodiode 25 can be equipped on the samesubstrate 26, thereby enabling miniaturization of the apparatus.

The monitoring optical system can include, as shown in FIG. 11, a curvedmirror 28 having an opening and the imaging lens 24. The curved mirror28 has a positive power. In this case, the light source 14 a and thephotodiode 25 can be equipped on the same substrate 26, and the numberof reflection of the monitoring light can be made only once.Accordingly, beam-spot position errors in the photodiode 25 can bereduced, and the photodiode 25 can be made smaller. Accordingly, lowercost and smaller size of the apparatus can be realized. Because thereflecting mirror 27 is not required, the number of parts can bereduced. In this case, the coupling lens 15 and the imaging lens 24 canbe integrated. At this time, the coupling lens 15 and the imaging lens24 can be one resin molded article.

In the first embodiment, the polygon mirror is used as a deflector;however, other types of mirrors, for example, a vibrating mirror canalso be used as a deflector. The vibrating mirror is a micro mirror thatcauses sine vibration of a resonant structure, using, for example,micromachine technology. Accordingly, the apparatus can be miniaturized,and banding, temperature increase, noise, power consumption, and thelike can be reduced considerably.

When the vibrating mirror is used as the deflector, the optical sagoccurring at the time of deflecting the light can be reduced than in thepolygon mirror. It is because the rotation center of the vibratingmirror is positioned substantially on the deflection-reflecting surface,and the sag does not occur or becomes very small even when a deflectionangle changes. Accordingly, a change of the space between the respectivebeams in the sub-scanning direction on the deflection-reflecting surfacecan be reduced over the whole image height. As a result, thesub-scanning beam pitch deviation on the photosensitive drum can befurther reduced.

The deflection-reflecting surface of the vibrating mirror is very small;however, it is applicable because the distance between two lightemitting units on the opposite sides in the main scanning direction issmall in the two-dimensional array 100. As for the sub-scanningdirection, the beams are narrowed down by the cylindrical lens and thelike, and the space between respective beams is set to be sufficientlysmall to obtain a desired sub-scanning beam pitch on the photosensitivedrum.

As shown in FIGS. 12 to 14, instead of the two-dimensional array 100, atwo-dimensional array (two-dimensional array 100A) can be used, in whichpart of the semiconductor layers in the two-dimensional array 100 ismade of different materials. In the two-dimensional array 100A, thespacer layer 113 in the two-dimensional array 100 is replaced by a spacelayer 213, the active layer 114 is replaced by an active layer 214, andthe spacer layer 115 is replaced by a spacer layer 215.

The spacer layer 213 is an (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P layerhaving a wide bandgap.

The active layer 214 includes, as shown in FIG. 13, three GaInPAsquantum well layers 214 a having a composition in which a compressivestrain remains and having a band gap wavelength of 780 nanometers andfour Ga_(0.6)In_(0.4)P barrier layers 214 b having a tensile strain,which is lattice-matched with the quantum well layers.

The spacer layer 215 is an (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P layerhaving a wide bandgap.

A part formed of the spacer layer 213, the active layer 214, and thespacer layer 215 is referred to as a resonator structure, and thethickness thereof is set to one wavelength optical thickness (see FIG.13).

In the two-dimensional array 100A, an AlGaInP material is used for thespacer layer. Therefore, a bandgap difference between the spacer layerand the active layer can be made larger than that of the two-dimensionalarray 100 of the first embodiment.

FIG. 14 is a table of bandgap difference between the spacer layer andthe quantum well layer and between the barrier layer and the quantumwell layer of VCSELs. The VCSELs include a VCSEL in which the materialof the spacer layer/quantum well layer is AlGaAs/AlGaAs and thewavelength thereof is 780 nanometers (hereinafter, “VCSEL_A.”), a VCSELin which the material of the spacer layer/quantum well layer isAlGaInP/GaInPAs and the wavelength thereof is 780 nanometers(hereinafter, “VCSEL_B”), and a VCSEL in which the material of thespacer layer/quantum well layer is AlGaAs/GaAs and the wavelengththereof is 850 nanometers (hereinafter, “VCSEL_C”). The VCSEL_Acorresponds to the VCSEL 101 in the two-dimensional array 100, and theVCSEL_B with x=0.7 corresponds to the VCSEL in the two-dimensional array100A (VCSEL 101A).

It is seen that the VCSEL_B can have a larger bandgap difference thanthat of the VCSEL_A and the VCSEL_C. Specifically, the bandgapdifference between the spacer layer and the quantum well layer is 767.3megaelectron volts (meV), which is quite larger than 465.9 meV of theVCSEL_A. Further, the VCSEL_B has a significant bandgap differencebetween the barrier layer and the quantum well layer as well, enablingexcellent carrier confinement.

Because the VCSEL 101A has a compressive strain in the quantum welllayer, a gain largely increases due to a band separation between a heavyhole and a light hole, thereby enabling high gain, and therefore highoutput with a low threshold. Accordingly, a reflectivity of thereflecting mirror on a light extracting side (the upper reflectingmirror 117) can be decreased, thereby enabling higher output. Further,because the high gain can be achieved, an optical output decrease due toa temperature rise can be suppressed, and the space between respectiveVCSELs in the two-dimensional array can be further narrowed down.

Further, in the VCSEL 101A, because the quantum well layer 214 a and thebarrier layer 214 b are made of a material, which does not containaluminum (Al), oxygen uptake to the active layer 214 can be reduced. Asa result, formation of a non-radiative recombination center can besuppressed, thereby enabling a longer life.

For example, when the two-dimensional array of the VCSELs is used for aoptical writing unit, if the service life of the VCSEL is short, theoptical writing unit is wasted. However, because the VCSEL 101A has along life, the optical writing unit using the two-dimensional array 100Acan be reused. Accordingly, resource protection is facilitated andenvironmental loads can be reduced. This is also applicable to otherapparatuses using the two-dimensional array of the VCSEL.

In the first embodiment, the light emitting units emits laser beamshaving a wavelength of 780 nanometers; however, the wavelength needsonly to correspond to sensitivity characteristics of the photosensitivedrum 1030. In this case, at least part of the material constituting eachlight emitting unit, or at least part of the configuration of each lightemitting unit is changed according to the emission wavelength.

In the first embodiment, the image forming apparatus is explained as aprinter; however, any image forming apparatus including the opticalscanning device 1010 can form a high quality image at a high speed.

A modification of the first embodiment is explained next with referenceto FIGS. 15 to 18. FIG. 15 is a schematic diagram of a printer 1000A asan image forming apparatus according to the modification of the firstembodiment.

The printer 1000A is a tandem multicolor-image forming apparatus thatsuperimposes four color images (black, cyan, magenta, and yellow) toform a full-color image. The printer 1000A includes an optical scanningdevice 1010A, photosensitive drums (30 a, 30 b, 30 c, and 30 d),chargers (32 a, 32 b, 32 c, and 32 d), developing rollers (33 a, 33 b,33 c, and 33 d), toner cartridges (34 a, 34 b, 34 c, and 34 d), cleaningcases (31 a, 31 b, 31 c, and 31 d), a transfer belt 40, a feed tray 60,a feed roller 54, a pair of registration rollers 56, a fuser roller 50,an eject tray 70, an eject roller 58, and a controller (not shown) thatcontrols the respective units.

As shown in FIG. 16, the optical scanning device 1010A includes lightsource units (200 a and 200 b), aperture plates (201 a and 201 b), lightdividing prisms (202 a and 202 b), cylindrical lenses (204 a, 204 b, 204c, and 204 d), a polygon mirror 104, fθ lenses (105 a, 105 b, 105 c, and105 d), folding mirrors (106, 106 b, 106 c, 106 d, 108 a, 108 b, 108 c,and 108 d), toroidal lenses (107 a, 107 b, 107 c, and 107 d), convergentlenses (156 a and 156 b), and photodetectors (157 a and 157 b).

Each of the light source unit 200 a and 200 b includes a light sourcehaving the two-dimensional array 100 or the two-dimensional array 100A,and a coupling lens that substantially collimates the light emitted fromthe light source.

The aperture plate 201 a has an aperture for specifying a beam diameterof the light from the light source 200 a. The aperture plate 201 b hasan aperture for specifying a beam diameter of the light from the lightsource 200 b. Because the aperture plates use the light reflected aroundthe aperture for monitoring, the aperture plates are arranged inclinedwith respect to the corresponding light source unit. Accordingly, it canbe prevented that the light reflected around the aperture returns to thelight source unit.

The light dividing prism 202 a divides the light having passed throughthe aperture of the aperture plate 201 a two beams parallel to eachother with a predetermined interval in the Z-axis direction. The lightdividing prism 202 b divides the light having passed through theaperture of the aperture plate 201 b into two beams parallel to eachother with a predetermined interval in the Z-axis direction.

The cylindrical lens 204 a is arranged on the optical path of a beam ona +/− side of the two beams from the light dividing prism 202 a(hereinafter, “black beam”), and converges the black beam near thedeflection-reflecting surface of the polygon mirror 104 in thesub-scanning direction.

The cylindrical lens 204 b is arranged on the optical path of a beam ona +Z side of the two beams from the light dividing prism 202 a(hereinafter, “cyan beam”), and converges the cyan beam near thedeflection-reflecting surface of the polygon mirror 104 in thesub-scanning direction.

The cylindrical lens 204 c is arranged on the optical path of the beamon the +Z side of the two beams from the light dividing prism 202 b(hereinafter, “magenta beam”), and converges the magenta beam near thedeflection-reflecting surface of the polygon mirror 104 in thesub-scanning direction.

The cylindrical lens 204 d is arranged on the optical path of the beamon the −Z side of the two beams from the light dividing prism 202 b(hereinafter, “yellow beam”), and converges the yellow beam near thedeflection-reflecting surface of the polygon mirror 104 in thesub-scanning direction.

The polygon mirror 104 has a four-sided mirror of a two-stage structure,and each mirror becomes the deflection-reflecting surface. The polygonmirror 104 is arranged so that the light from the cylindrical lens 204 aand the light from the cylindrical lens 204 d are deflected by thedeflection-reflecting surface on a first stage (lower stage), and thelight from the cylindrical lens 204 b and the light from the cylindricallens 204 c are deflected by the deflection-reflecting surface on asecond stage (upper stage). The first deflection-reflecting surface andthe second deflection-reflecting surface rotate with the phase beingshifted by 45° from each other, and the scanning of the light isperformed alternatively on the first and the second stages.

The fθ lenses 105 a and 105 b are arranged on a −X side of the polygonmirror 104, and the fθ lenses 105 c and 105 d are arranged on a +X sideof the polygon mirror 104. That is, bi-directional scanning is employed.

The fθ lenses 105 a and 105 b are laminated in the Z-axis direction, andthe fθ lens 105 a faces the first-stage deflection-reflecting surfaceand the fθ lens 105 b faces the second-stage deflection-reflectingsurface. The fθ lenses 105 c and 105 d are laminated in the Z-axisdirection, and the fθ lens 105 c faces the second-stagedeflection-reflecting surface and the fθ lens 105 d faces thefirst-stage deflection-reflecting surface.

The black beam deflected by the polygon mirror 104 enters the fθ lens105 a, the yellow beam enters the fθ 105 d, the cyan beam enters the fθlens 105 b, and the magenta beam enters the fθ lens 105 c.

In the optical scanning device 1010A, as shown in FIG. 17A, a beam Bkfrom the cylindrical lens 204 a and a beam Bc from the cylindrical lens204 b are set to enter the polygon mirror 104 as being inclined to the−Z side with respect to the normal direction RH of thedeflection-reflecting surface of the polygon mirror 104. A beam Bm fromthe cylindrical lens 204 c and a beam By from the cylindrical lens 204 dare set to enter the polygon mirror 104 as being inclined to the +Z sidewith respect to the normal direction RH of the deflection-reflectingsurface of the polygon mirror 104. That is, the beams incident to the −Xside of the polygon mirror 104 and the beams incident to the +Z side areinclined to the opposite sides with respect to the directionperpendicular to the polygon mirror 104 at least in the sub-scanningdirection.

The angle of inclination of the beam Bk from the cylindrical lens 204 aand that of the beam Bc from the cylindrical lens 204 b are equal toeach other, and the angle of inclination of the beam Bm from thecylindrical lens 204 c and that of the beam By from the cylindrical lens204 d are equal to each other.

The size of the angle of inclination of the beam Bk from the cylindricallens 204 a and that of the beam Bc from the cylindrical lens 204 b, andthe size of the angle of inclination of the beam Bm from the cylindricallens 204 c and that of the beam By from the cylindrical lens 204 d areequal to each other. The angle of inclination is set here to a range offrom 0.5 degree to 1.0 degree as one example.

The black beam transmitted through the fθ lens 105 a is imaged in a spoton the photosensitive drum 30 a via the folding mirror 106 a, thetoroidal lens 107 a, and the folding mirror 108 a (see FIG. 18).

The cyan beam transmitted through the fθ lens 105 b is imaged in a spoton the photosensitive drum 30 b via the folding mirror 106 b, thetoroidal lens 107 b, and the folding mirror 108 b (see FIG. 18).

The magenta beam transmitted through the fθ lens 105 c is imaged in aspot on the photosensitive drum 30 c via the folding mirror 106 c, thetoroidal lens 107 c, and the folding mirror 108 c (see FIG. 18).

The yellow beam transmitted through the fθ lens 105 d is imaged in aspot on the photosensitive drum 30 d via the folding mirror 106 d, thetoroidal lens 107 d, and the folding mirror 108 d (see FIG. 18).

According to the modification of the first embodiment, the two lightsources each include the two-dimensional array 100 or thetwo-dimensional array 100A, and the beams from the two light sources areset to enter the polygon mirror, inclined with respect to the normaldirection of the deflection-reflecting surface in the sub-scanningdirection. Accordingly, the same effects as with the optical scanningdevice 1010 can be obtained.

The printer 1000A including the optical scanning device 1010A is capableof forming a high-quality multicolor image at a high speed.

In modification of the first embodiment, the optical scanning device1010A is explained as being of bi-directional scanning type; however, asshown in FIG. 19, the optical scanning device can be of one-directionalscanning type. The optical scanning device shown in FIG. 19 includes anfθ lens 105, and folding mirrors 108 c ₁ and 108 c ₂.

In this case, the beam Bk from the cylindrical lens 204 a, the beam Bcfrom the cylindrical lens 204 b, the beam Bm from the cylindrical lens204 c, and the beam By from the cylindrical lens 204 d enter one side(+X side in FIG. 19) of the polygon mirror 104. At this time, the beamsenter substantially the same position on the deflection-reflectingsurface in the sub-scanning direction, and the angles of inclination ofthe respective beams are different from each other.

Also in this case, the same effects as those of the optical scanningdevice 1010A can be obtained.

Further, in this case, because the deflection-reflecting surface of thepolygon mirror can be formed on one stage and the thickness in thesub-scanning direction can be reduced as compared to the case of theoptical scanning device 1010A, an inertia as a rotator is small, therebyenabling to shorten start-up time. Accordingly, a low-cost opticalscanning device with low power consumption can be realized. In addition,because a scanning optical system is provided only on one side of thepolygon mirror, there is no need to consider the effect of a ghostlight.

In this case, if the thickness of the deflection-reflecting surface inthe sub-scanning direction can be increased, as shown in FIG. 20, thebeams can be made to enter positions different from each other on thedeflection-reflecting surface in the sub-scanning direction. At thistime, the angle of inclination of the beams can be made the same.

In modification of the first embodiment, the vibrating mirror can beused instead of the polygon mirror 104.

FIG. 21 is a schematic diagram of an optical scanning device 1010Baccording to a second embodiment of the present invention. The opticalscanning device 1010B includes a light source 14 b, the coupling lens15, the aperture plate (aperture member) 23, the cylindrical lens(imaging lens) 17, and a reflecting mirror 14 sequentially arranged inthe −Y direction from the light source 14 b, a polygon mirror 13 barranged on the −X side of the reflecting mirror 14, and thedeflector-side scanning lens 11 a (first scanning lens) and theimage-surface side scanning lens 11 b (second scanning lens)sequentially arranged on the +X side of the reflecting mirror 14.

The light source 14 b is a VCSEL array, for example, in which the VCSELsare two-dimensionally arranged as a source of luminescence, and as shownin FIG. 22, 40 VCSELs are arranged in a matrix of eight rows and 5columns, in which a direction parallel to a straight line L forming anangle θ with the Y-axis is designated as a row direction, and adirection parallel to the Z axis is designated as a column direction. Ineach VCSEL, a diameter of a near-field pattern is 4 micrometers, andlight beams having a wavelength of 780 nanometers are emitted with adivergence angle in the main and sub-scanning directions being 7±1degrees. In the second embodiment, a row interval Dz is 24.0micrometers, and a column interval Dy is 23.9 micrometers. An intervaldz between adjacent VCSELs in the Z-axis direction (sub-scanningdirection) is 4.8 micrometers (=Dz/5). In the explanations below, asshown in FIG. 3, the VCSEL positioned in the m-th row and the n-thcolumn is referred to as VCSEL_(mn).

FIG. 23 is a sectional view of a 780 nm-band VCSEL. FIG. 24 is anenlarged view around an active layer 124 of the VCSEL. As can be seenfrom FIGS. 23 and 24, the VCSEL has a configuration in which a resonatorregion having an optical thickness of one wavelength and including theactive layer 124 including an Al₀₁₂Ga_(0.88)As quantum well layer 124 aand an Al_(0.3)Ga_(0.7)As barrier layer 124 b, and Al_(0.6)Ga_(0.4)Asspacer layers 123 and 125 is put between a lower reflecting mirror 122including 40.5 pairs of n-Al_(0.3)Ga_(0.7)As high refractive indexlayer/n-Al_(0.9)Ga_(0.1)As low refractive index layer and an upperreflecting mirror 127 including 124 pairs of p-Al_(0.3)Ga_(0.7)As highrefractive index layer/p-Al_(0.9)Ga_(0.1)As low refractive index layer,with an optical thickness of λ/4 in each layer, on an n-GaAs substrate121 on which the n-side common electrode 110 is formed. An AlAs selectedoxide layer 130 surrounded by an Al_(x)O_(y) current constriction layer126 is provided on the upper reflecting mirror 117 at a position λ/4away from the resonator region. A composition gradient layer (notshown), in which the composition is gradually changed, is providedbetween the reflecting mirrors 122 and 127 for reducing electricalresistance.

A forming method of the VCSEL provided on the light source 14 b isexplained. The layers are created according to the crystal growth usingthe metal organic chemical vapor deposition (MOCVD method) or themolecular beam epitaxy (MBE method).

A mesa shape is formed by forming a slot having a depth of, for example,4.5 micrometers around a region, which becomes a device region, by dryetching. The etching bottom is generally provided at a positionexceeding at least the AlAs selected oxide layer 130.

The AlAs selected oxide layer 130, whose side is exposed by the slotforming process by etching is heat treated in water vapor to oxidize theperiphery thereof and change the peripheral part to an Al_(x)O_(y)insulator layer, thereby forming the current constriction structure inwhich the route of element drive currents is restricted only to theunoxidized AlAs region at the central part.

An SiO₂ protective layer (not shown) having a thickness of, for example,150 nanometers is provided excluding a region where an upper electrode(p-side individual electrode) 131 is formed on each device region and anlight emitting unit 132, and the etching portion is buried withpolyimide 129 for planarization.

The polyimide and the SiO₂ protective layer (not shown) on a p-contactlayer 128 in each device region and the upper reflecting surfaceincluding the light emitting unit are removed and the p-side individualelectrode 131 is formed, excluding the light emitting unit 132 on thep-contact layer 128, and an n-side electrode is formed on a lowersurface of the n-GaAs substrate 121.

In the second embodiment, the mesa formed by dry etching becomes eachVCSEL. To arrange the sources of luminescence of the light source 14 b,a photo mask is formed along the arrangement of the sources ofluminescence, an etching mask is then formed by a normalphotolithographic process, and etching is performed. It is preferredthat the slot of 4 micrometers to 5 micrometers or larger is providedbetween the devices for electrical and spatial separation of respectivedevices in the array. It is because if the slot is too narrow, etchingcontrol becomes difficult. The mesa shape can be an arbitrary shape suchas oval, square, rectangular other than circular in the secondembodiment. It is preferred that the size (diameter and the like) of themesa is about 10 micrometers or larger. It is because if the size is toosmall, heat is accumulated at the time of operation of the device,thereby deteriorating the characteristic.

Such 780 nm-band VCSEL can be made of another material. FIG. 25 is anenlarged view around the active layer 124 made of another material. Theactive layer 124 includes three GaInPAs quantum well active layers 124 chaving a compressive strain composition and a bandgap wavelength of 780nanometers, and four Ga_(0.6)In_(0.4)P barrier layers 124 d having atensile strain, which lattice-match with the quantum well active layers124 c, and an (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P, which is a widebandgap, is used as cladding layers 123 and 125 (spacer layers in thesecond embodiment) for carrier confinement. In this case, the bandgapdifference between the cladding layers and the quantum well active layercan be made quite large, as compared with a case that the claddinglayers for carrier confinement are made of AlGaAs.

FIG. 14 depicts bandgap difference between the spacer layer and thequantum well layer and between the barrier layer and the quantum welllayer of the AlGaAs/AlGaAs (spacer layer/quantum well layer) 780-nm and850-nm VCSELs and the AlGaInP/GaInPAs (spacer layer/quantum well layer)780-nm VCSEL. The spacer layer stands for a layer between the activelayer and the reflecting mirror in the general configuration, and has afunction as the cladding layer for confining the carriers.

As shown in FIG. 14, with the AlGaInP/GaInPAs (spacer layer/quantum welllayer) 780-nm VCSEL, the bandgap difference can be made larger than thatof the AlGaAs/AlGaAs 780-nm VCSEL and the AlGaInP/GaInPAs 850-nm VCSEL.Specifically, the bandgap difference between the cladding layer and theactive layer is as large as 767 meV, as compared with 466 meV (when thecladding layer is made of AlGaAs (Al composition is 0.6). The bandgapdifference between the barrier layer and the active layer is also large,which enables excellent carrier confinement.

Further, because the active layer has the compressive strain, a gainincrease becomes large due to a band separation between a heavy hole anda light hole, thereby enabling high gain, and therefore high output witha low threshold. This effect cannot be obtained by the 780-nm or 850-nmVCSEL made of AlGaAs system, having substantially the same latticeconstant as the GaAs substrate. Further, the threshold can be set low byimproving the carrier confinement and achieving high gain by the strainquantum-well active layer, thereby enabling to decrease the reflectivityof a distributed Bragg reflector (DBR) on the light extracting side, andobtain high output.

The active layer and the barrier layer are made of a material notcontaining Al, to form an Al-free active region (the quantum well activelayer and the adjacent layer thereto). Therefore, oxygen uptake to theactive layer can be reduced, and formation of the non-radiativerecombination center can be suppressed, thereby enabling a long life.Accordingly, the write unit or the light source unit can be reused.

Referring back to FIG. 21, the coupling lens 15 has a focal length of47.7 millimeters, and substantially collimates the light beams emittedfrom the light source 14 b.

The aperture plate (aperture member) 23 has a rectangular or ovalaperture having a size in the Y-axis direction (main scanning direction)of 5.44 millimeters, and a size in the Z-axis direction (sub-scanningdirection) of 2.10 millimeters. The aperture is arranged so that thecenter of the aperture is positioned at the focal position or near thefocal position of the coupling lens 15.

The cylindrical lens (imaging lens) 17 has a focal length of 107.0millimeters and a refracting power in the Z-axis direction (sub-scanningdirection). The cylindrical lens 17 images the light beams transmittedthrough the aperture plate (aperture member) 23 in the sub-scanningdirection, near the deflection-reflecting surface of the polygon mirror13 b via the reflecting mirror 18.

The polygon mirror 13 b is a member having a shape of a regularpolygonal column, which is a regular dodecagon, whose upper face touchesinternally with a circle having a radius of 7 millimeters. The deflectorsurfaces that deflect the incident light beams are formed on the sidesof a dodecahedron of the polygon mirror 13 b, and rotated by a rotationmechanism (not shown) at a constant angular velocity around a rotationaxis parallel to the Z-axis direction. Accordingly, the light beamsincident to the polygon mirror 13 b are scanned in the Y-axis direction.

The deflector-side scanning lens (first scanning lens) 11 a and theimage-surface side scanning lens (second scanning lens) 11 b are, forexample, resin scanning lenses having a thickness of 13.5 millimetersand 3.5 millimeters, respectively, at the center (on the optical axis).

In the optical scanning device 1010B, sub-scanning lateral magnificationas the whole optical scanning device 1010B is 2.18 times, and thesub-scanning lateral magnification of the optical system (scanningoptical system) after the polygon mirror 13 b is 0.97 times. The focallength of the scanning optical system in the main scanning direction is237.8 millimeters, and the focal length thereof in the sub-scanningdirection is 71.4 millimeters. A width of a writing area of thephotosensitive drum 1030 is in a range of ±105.0 millimeters in the mainscanning direction (Y-axis direction), centering on a point O shown inFIG. 21. The point O is a point where a straight line parallel to the Xaxis passing through the rotation center of the polygon mirror 13 b andthe surface to be scanned of the photosensitive drum 1030 cross eachother in FIG. 21. The spot diameter of the light beams on the surface ofthe photosensitive drum 1030 is desirably 52 micrometers in the mainscanning direction and 55 micrometers in the sub-scanning direction. Asshown in FIG. 21, optical distances d1, d2, d3, d4, d5, d6, d7, and d8between the light source 14 b and the optical devices, and sizes D1, D2,D3, and D4 of the respective devices in the optical axis direction areas shown in Tables 1 and 2 as one example.

TABLE 1 Unit: mm d1 d2 d3 d4 d5 d6 d7 d8 46.06 10.32 56.5 40.0 13.046.31 89.73 141.36

TABLE 2 Unit: mm D1 D2 D3 D4 3.0 3.0 13.50 3.50

FIG. 26A depicts the intensity of the light beams emitted from a VCSEL₁₃positioned at the center of the first row, and a VCSEL₁₁ and a VCSEL₁₅positioned at the opposite ends in the main scanning direction of thefirst row, among the VCSELs in the light source 14 b, and incident to asurface parallel to the deflector surface of the polygon mirror 13 b(hereinafter, “plane of incidence”). A position coordinate in the mainscanning direction on the plane of incidence is plotted on thehorizontal axis in FIG. 26A is, and the intensity of the light beams isplotted on the vertical axis, where y2 is a position coordinate at thecenter in the main scanning direction on the deflector surface of thepolygon mirror 13 b, and y1 and y3 are position coordinates at theopposite ends in the main scanning direction on the deflector surface.The light beams emitted from the VCSEL₁₁, VCSEL₁₃, and VCSEL₁₅ arereferred to as light beams LB₁₁, LB₁₃, and LB₁₅, respectively.

When scanning near the center point O in the writing area of thephotosensitive drum 1030 is performed by the light beams LB₁₁, LB₁₃, andLB₁₅ having equal intensity, because the light beams LB₁₁, LB₁₃, andLB₁₅ are deflected by the deflector surface substantially parallel to aZY plane, intensity distributions in the planes of incidence of therespective light beams LB₁₁, LB₁₃, and LB₁₅ are indicated by curvedlines L1, L2, and L3 in FIG. 26A. Because only part of two light beamsLB₁₁ and LB₁₅ of the three light beams LB₁₁, LB₁₃, and LB₁₅ is incidentto the deflector surface, the quantities of light after being reflectedby the deflector surface are proportional to areas of regions A1 and A3(hatched region in FIG. 26A) specified by the curved line L1 or L3, astraight line passing through the position y1 or y2 and perpendicular tothe Y axis, and the Y axis. On the other hand, as for the light beamsLB₁₃, because all the light beams LB₁₃ are incident to the deflectorsurface, the quantity of light after being reflected by the deflectorsurface is proportional to an area of a region A2 specified by thecurved line L2 and the Y axis. That is, the ratio of the quantity oflight beams LB₁₁, LB₁₃, and LB₁₅ reflected by the deflector surface isequal to the area ratio of regions A1, A2, and A3, and increases towardthe light beams incident to the center of the deflector surface, anddecreases toward the light beams incident to the vicinity of an edge atthe opposite ends of the deflector surface in the main scanningdirection.

Accordingly, in the optical scanning device 1010B according to thesecond embodiment, when scanning near the center point O in the writingarea of the photosensitive drum 1030 is performed, the VCSEL₁₁, VCSEL₁₃,and VCSEL₁₅ are driven so that the intensity of the two light beams LB₁₁and LB₁₅ is stronger than that of the light beam LB₁₃. For example, asshown in FIG. 26B, the VCSEL₁₁, VCSEL_(n), and VCSEL_(n) are driven sothat peak values of the two curved lines L1 and L2 is larger than a peakvalue of the curved line L2, and the area of the regions A1, A2, and A3is substantially equal to each other. Accordingly, the quantities oflight of the three light beams LB₁₁, LB₁₃, and LB₁₅ reflected by thedeflector surface become equal to each other.

The VCSEL₁₂ and VCSEL₁₄ arranged on the first row in the light source 14b are also controlled so that the intensity of light beams to be emittedtherefrom becomes stronger than that of the light beam LB₁₃, and thequantities of light beams reflected by the deflector surface becomeequal to the quantity of light of the light beam LB₁₃, L reflected bythe deflector surface. The VCSEL₂₁ to VCSEL₈₅ arranged on the second tothe eighth rows are also controlled in the same manner as in the VCSELon the first row, so that the intensity of the light beams to be emittedfrom the VCSELs arranged at the opposite ends in the main scanningdirection becomes stronger than the intensity of the light beam to beemitted from the VCSEL_(m3) arranged at the center in the main scanningdirection. Accordingly, the quantities of light beams emitted from theVCSELs of the light source 14 b and deflected by the deflector surfaceof the polygon mirror 13 b become substantially equal to each other.

The light beams deflected by the deflector surface of the rotatingpolygon mirror 13 b are focused on the surface of the photosensitivedrum 1030, in a state with a moving speed in the main scanning directionof the light beam spot being adjusted by the deflector-side scanninglens (the first scanning lens) 11 a and the image-surface side scanninglens (the second scanning lens) 11 b.

On the other hand, the surface of the photosensitive drum 1030 ischarged with a predetermined voltage by the charger 1031, and thereforecharges are distributed with a certain charge density. When thephotosensitive drum 1030 is scanned by the light beams deflected by thepolygon mirror 13 b, carriers (charges) are generated in aphotosensitive layer to which the light beams are incident, and in thisportion, charge transfer occurs and the potential decreases. Therefore,because the photosensitive drum 1030 rotating in a direction of arrow inFIG. 1 is scanned by the light beams modulated based on imageinformation, an electrostatic latent image regulated by the chargedistribution is formed on the surface thereof. Thereafter, a developingprocess, paper feed and transport, a transfer process, and a fixingprocess are performed.

In the optical scanning device 1010B according to the second embodiment,because the quantities of light beams deflected by the deflector surfacebecome substantially uniform, the scanning of the writing area on thephotosensitive drum 1030 is performed by the light beams having the samequantity of light. Accordingly, the whole area of the writing area canbe uniformly scanned.

As shown in FIG. 22, for example, the 40 VCSELs in the light source 10are two-dimensionally arranged on the surface parallel to the ZY planeso that the distance (=148.8 μm) between the VCSELs farthest apart fromeach other in the sub-scanning direction (Z-axis direction) is largerthan the distance (=95.6 μm) between the light emitting units farthestapart from each other in the main scanning direction (Y-axis direction).Accordingly, non-uniform distribution of the quantity of light of thelight beams deflected by the deflector surface of the polygon mirror 13b can be avoided, and the surface to be scanned can be scanned highlyaccurately.

In the optical scanning device 1010B according to the second embodiment,when the writing area on the −Y side than the central part of thewriting area (near the center point O) of the photosensitive drum 1030is scanned, for example, as shown in FIG. 27A, the incident position ofthe light beams on the deflector surface moves to the +Y direction.Accordingly, the quantity of light of the light beams, which is notincident to the deflector surface, increases on the +Y side. In thiscase, the intensity of the light beams to be incident to the vicinity ofthe +Y side edge on the deflector surface is increased than that of thelight beam L2, like the light beam L3 as shown in FIG. 28A, to drive theVCSELs in the light source 14 b so that the areas of the regions A3 andA2 become substantially the same. Accordingly, the light beams havinguniform quantity of light can scan from the central part of the writingarea to the −Y side writing area.

When the writing area on the +Y side than the central part of thewriting area (near the center point O) is scanned, as shown in FIG. 27B,the incident position of the light beams on the deflector surface movesto the −Y direction. Accordingly, the quantity of light of the lightbeams, which is not incident to the deflector surface, increases on the−Y direction. In this case, the intensity of the light beams to beincident to the vicinity of the −Y side edge on the deflector surface isincreased than that of the light beam L1 as shown in FIG. 28B, to drivethe VCSELs in the light source 14 b so that the areas of the regions A1and A2 become substantially the same. Accordingly, the light beamshaving uniform quantity of light can scan from the central part of thewriting area to the −Y side writing area.

The spots of light beams can be superimposed on the deflector surface ofthe polygon mirror 13 b by an anamorphic lens having a refracting powerin the main scanning direction for the cylindrical lens (imaging lens)17. When the anamorphic lens having, for example, the focal length inthe sub-scanning direction of 107.0 millimeters and the focal length inthe main scanning direction of 54 millimeters is used for thecylindrical lens (imaging lens) 17 in the optical system shown in FIG.21, as shown in FIG. 29, the light beams emitted from the light source14 b is focused near a certain point on the deflector surface formed onthe polygon mirror 13 b. In this case, because the curved linesindicating intensity distribution of light beams are substantiallysuperimposed on each other, as shown in FIG. 30, a plurality of lightbeams having the uniform intensity is emitted from the light source 14b, thereby enabling to scan the writing area on the photosensitive drum1030 by the light beams having the uniform quantity of light.

In the second embodiment, the aperture plate 23 has a rectangular oroval aperture with a size of 5.44 millimeters in the Y-axis direction(main scanning direction), and 2.10 millimeters in the Z-axis direction(sub-scanning direction). However, the aperture plate 23 can be arrangedat a position slightly shifted in the Y-axis direction from the focalpoint of the coupling lens 15, and, as shown in FIG. 31, the aperturecan have a shape such that the size in the sub-scanning directionincreases from the center to the opposite ends in the main scanningdirection. Accordingly, the quantity of light of the light beamsincident to the vicinity of the edges at the opposite ends of thepolygon mirror 13 b in the main scanning direction can be increased thanthat of the light beams incident to the center of the deflector surfacein the main scanning direction. As a result, the quantity of light ofthe light beams reflected by the deflector surface can be made uniform.

With the image forming apparatus 1000 according to the second embodimentincluding the optical scanning device 1010B, an image is obtained from alatent image formed on the photosensitive drum 201 with light beamshaving the uniform quantity of light. Accordingly, an image can beformed on a sheet highly accurately.

In the second embodiment, the optical scanning device 1010B is explainedas being applied to the monochrome image forming apparatus 1000;however, it can be applied to a tandem color image forming apparatus,such as a color copier, capable of forming a full-color image andincluding a plurality of photosensitive drums.

A multicolor-image forming apparatus 300 capable of forming a colorimage and including a plurality of photosensitive drums is explainedwith reference to FIGS. 32 to 34. The multicolor-image forming apparatus300 includes a photosensitive drum K1, a charger K2, a developing unitK4, a cleaning unit K5, and a transfer changer K6 for black; aphotosensitive drum C1, a charger C2, a developing unit C4, a cleaningunit C5, and a transfer changer C6 for cyan; a photosensitive drum M1, acharger M2, a developing unit M4, a cleaning unit M5, and a transferchanger M6 for magenta; a photosensitive drum Y1, a charger Y2, adeveloping unit Y4, a cleaning unit Y5, and a transfer changer Y6 foryellow; an optical scanning device 900; a transfer belt 901; and a fuser902.

The photosensitive drums K1, C1, M1, and Y1 rotate in a direction ofarrow shown in FIG. 32, and the charger (K2, C2, M2, and Y2), thedeveloping unit (K4, C4, M4, and Y4), the transfer charger (K6, C6, M6,and Y6), and the cleaning unit (K5, C5, M5, and Y5) are arranged in anorder of rotation. The chargers K2, C2, M2, and Y2 uniformly charge thesurface of the corresponding photosensitive drums K1, C1, M1, and Y1.The optical scanning device 900 irradiates the photosensitive drums K1,C1, M1, and Y1 charged by the charger K2, C2, M2, and Y2 with light,thereby forming an electrostatic latent image on the surface of each ofthe photosensitive drums K1, C1, M1, and Y1. The electrostatic latentimage is developed into a toner image by corresponding one of thedeveloping units K4, C4, M4, and Y4. The toner images of respectivecolors are superimposed to form a full-color image, and transferred ontoa sheet by the transfer chargers K6, C6, M6, and Y6. The full-colorimage is fixed on the sheet by the fuser 902.

The optical scanning device 900 is explained next with reference toFIGS. 33 and 34.

The optical scanning device 900 includes light source units 10K, 10C,10M, and 10Y, an optical system (not shown) that includes a couplinglens 11 and an aperture member 12 as in the optical scanning device1010, and guides the light beams emitted from the light source units10K, 10C, 10M, and 10Y to a polygon mirror 230, the polygon mirror 230,first scanning lenses 218 a, 218 b, 218 c, and 218 d, folding mirrors224 a, 224 b, 224 c, 224 d, 227 a, 227 b, 227 c, and 227 d, and secondscanning lenses 220 a, 220 b, 220 c, and 220 d. Only part of the opticalscanning device 900 is shown in FIGS. 14 and 15 for convenience.

The light source units 10K, 10C, 10M, and 10Y include the light source10.

The light source unit 10K emits laser beams (hereinafter, “black beams”)modulated corresponding to black image information. The light sourceunit 10C emits laser beams (hereinafter, “cyan beams”) modulatedcorresponding to cyan image information. The light source unit 10M emitslaser beams (hereinafter, “magenta beams”) modulated corresponding tomagenta image information. The light source unit 10Y emits laser beams(hereinafter, “yellow beams”) modulated corresponding to yellow imageinformation.

The first scanning lens 218 a, the folding mirror 224 a, the secondscanning lens 220 a, and the folding mirror 227 a correspond to theblack beams.

The first scanning lens 218 b, the folding mirror 224 b, the secondscanning lens 220 b, and the folding mirror 227 b correspond to the cyanbeams.

The first scanning lens 218 c, the folding mirror 224 c, the secondscanning lens 220 c, and the folding mirror 227 c correspond to themagenta beams.

The first scanning lens 218 d, the folding mirror 224 d, the secondscanning lens 220 d, and the folding mirror 227 d correspond to theyellow beams.

The laser beams emitted from the light source units are converged in thesub-scanning direction so that the laser beams become linear on thedeflector surface of the polygon mirror 230. A deflection point on thepolygon mirror 230 and a focused point on the surface of thecorresponding photosensitive drum become conjugate in the sub-scanningdirection.

The polygon mirror 230 includes a six-sided mirror having a two-stagestructure. The black beams from the light source unit 10K and the yellowbeams from the light source unit 10Y are deflected by the six-sidedmirror on a first stage, and cyan beams from the light source unit 10Cand the magenta beams from the light source unit 10M are deflected bythe six-sided mirror on a second stage. That is, all the laser beams aredeflected by the single polygon mirror 230.

The first scanning lenses 218 a and 218 b are arranged on one side (+Xside) of the polygon mirror 230, and the first scanning lenses 218 c and218 d are arranged on the other side (−X side) of the polygon mirror230. The first scanning lenses 218 a and 218 b, and the first scanninglenses 218 c and 218 d are laminated in a direction corresponding to thesub-scanning direction (Z-axis direction).

The black beams from the first scanning lens 218 a is imaged in a spoton the photosensitive drum K1 via the folding mirror 224 a, the firstscanning lens 220 a, and the folding mirror 227 a.

The cyan beams from the first scanning lens 218 b is imaged in a spot onthe photosensitive drum C1 via the folding mirror 224 b, the firstscanning lens 220 b, and the folding mirror 227 b.

The magenta beams from the first scanning lens 218 c is imaged in a spoton the photosensitive drum M1 via the folding mirror 224 c, the firstscanning lens 220 c, and the folding mirror 227 c.

The yellow beams from the first scanning lens 218 d is imaged in a spoton the photosensitive drum Y1 via the folding mirror 224 d, the firstscanning lens 220 d, and the folding mirror 227 d.

The folding mirrors are arranged so that the optical path lengths fromthe polygon mirror 230 to the respective photosensitive drums match eachother and the incident position and the incident angle of the laserbeams on the respective photosensitive drums are equal to each other.

In the multicolor-image forming apparatus 300, a line image is formed onthe photosensitive drums K1, C1, M1, and Y1 by the light beams, with thequantity of light being uniformly adjusted. Accordingly, a highlydetailed multicolor image can be accurately formed on the recordingmedium.

In the above embodiments, the optical scanning device is explained asbeing applied to a printer; however, it can also be applied to othertypes of image forming apparatuses such as copiers and facsimilemachines, and multifunction products that combine any or all of thefunctions of these.

As set forth hereinabove, according to an embodiment of the presentinvention, it can be prevented that the light emitted from the lightsource and reflected by the deflection-reflecting surface returns to thelight source side, and therefore the quantity of light emitted from thelight source can be monitored accurately. Further, the sub-scanning beampitch deviation due to grazing incidence can be reduced. As a result,the surface to be scanned can be stably scanned by the beams.

Moreover, non-uniform distribution of the quantity of light of aplurality of light beams deflected by the deflector surface (deflectorsurface of the overfilled optical system) having a deflector surfacesmaller than the beam width of the light beams in the main scanningdirection can be avoided, and the surface to be scanned can be scannedhighly accurately.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

1. An optical scanning device comprising: a light source that includes aplurality of light emitting units for emitting light beams; a deflectorthat includes a deflecting surface for deflecting the light beams fromthe light source; a scanning optical system that focuses the light beamsdeflected by the deflector on a surface to be scanned; and a monitoringoptical system that monitors a quantity of light beams emitted from thelight source, wherein the light emitting units are arrangedtwo-dimensionally, and a distance between two light emitting units atopposite ends in a main scanning direction is smaller than a distancebetween two light emitting units at opposite ends in a sub-scanningdirection, the size of the deflecting surface in the main scanningdirection is smaller than a width of a light flux of light beams in themain scanning direction, and upon scanning around a center of a writingarea with light beams, intensities of the light beams increase from alight beam at a center in the main scanning direction toward at leastone of a light beam at a write-start position and a light beam at awrite-end position.
 2. The optical scanning device according to claim 1,wherein the light emitting units are arranged so that a distance betweenlight emitting units farthest apart from each other in the sub-scanningdirection is larger than a distance between light emitting unitsfarthest apart from each other in the main scanning direction.
 3. Theoptical scanning device according to claim 1, further comprising: acoupling lens that couples light beams to be incident to the deflector,and an aperture member that includes an aperture through which the lightbeams coupled by the coupling lens pass, wherein the aperture member isarranged so that the aperture is positioned near a focal position of thecoupling lens, and a width of the aperture in the sub-scanning directionincreases from center toward periphery in the main scanning direction.4. The optical scanning device according to claim 1, further comprising:a coupling lens that couples light beams to be incident to thedeflector; and an optical element that focuses the light beams coupledby the coupling lens on the deflecting surface of the deflector in thesub-scanning direction, wherein the optical element has a positive powerin the sub-scanning direction, and is located closer to the deflectorthan a focal position of the coupling lens.
 5. The optical scanningdevice according to claim 1, wherein the light beams enter the deflectorfrom a direction inclined with respect to a normal direction of thedeflecting surface at least in a sub-scanning direction.
 6. The opticalscanning device according to claim 1, further comprising: a rectangularor oval shaped aperture member.
 7. An optical scanning devicecomprising: a light source that includes a plurality of light emittingunits for emitting light beams; a deflector that includes a deflectingsurface for deflecting the light beams from the light source; a scanningoptical system that focuses the light beams deflected by the deflectoron a surface to be scanned; and a monitoring optical system thatmonitors a quantity of light beams emitted from the light source,wherein the light emitting units are arranged two-dimensionally, and adistance between two light emitting units at opposite ends in a mainscanning direction is smaller than a distance between two light emittingunits at opposite ends in a sub-scanning direction, the size of thedeflecting surface in the main scanning direction is smaller than awidth of a light flux of light beams in the main scanning direction,upon scanning from a write-start position to a center of a writing areawith light beams, intensities of the light beams incident to the writingarea increase from a light beam at the center in the main scanningdirection toward a light beam at a write-end position, and upon scanningfrom the center to the write-end position of the writing area with lightbeams, intensities of the light beams incident to the writing areaincrease from a light beam at the center in the main scanning directiontoward a light beam at the write-start position.